Semiconductor devices such as memories or microcomputers used for computers are manufactured by repeating the steps for transferring, using the exposure, lithography and etching processes, a pattern of circuits formed on a photomask. During the manufacturing process of semiconductor devices, the quality as a result of the lithography, etching and the other processes, and the presence of defects such as generation of foreign matters greatly affects the manufacturing yield of the semiconductor devices. Accordingly, to detect the presence of abnormality or defects at an early stage or even before they occur, an inspection of the patterns on the semiconductor wafer is performed upon termination of each manufacturing process. As one example of a method for inspecting defects existing in a pattern on the semiconductor wafer, a method and an apparatus for inspecting patterns using an electron-beam image have been put to practical use.
U.S. Pat. No. 6,583,414 B2 discloses an inspection apparatus including a section for irradiating and scanning an electron beam to a surface of a substrate where a circuit pattern is formed; a detecting section for detecting signals secondarily generated from the surface of the substrate; a section for forming an image on a display device from the signals detected by the detecting section; and a section for automatically recognizing information about foreign matters and defects based on figure, roughness and voltage contrast of the surface of the substrate from the image. These inspection apparatuses must obtain an image with a higher SN ratio at an extremely high speed in order to perform a high-throughput and high-accuracy inspection. To satisfy such a requirement, it is required to keep a higher SN ratio by ensuring the necessary number of electrons to be irradiated to the circuit pattern using a large-current electron beam which is equal to or more than 1000 times (100 nA or more) that used for a usual scanning electron microscope (SEM). Further, it is essential to highly efficiently detect secondary electrons produced from a substrate and reflection electrons reflected therefrom at a higher speed.
During the detection of defects, when the originally-unintended defects are detected by the noise caused by various reasons, it is expected that there is caused deterioration not only in the classification accuracy of the automatic defect recognition but also in the inspection performance itself of the inspection device. Therefore, as disclosed in Japanese Patent Application Laid Open No. 2002-124555, there is used a method in which defects are detected and then automatically classified to thereby determine whether the defects include a real defect or false information. Further, as disclosed in U.S. Pat. No. 7,230,723 B2, there is also used a method in which noise randomly generated is statistically indexed and reduction in the noise is performed by the image processing.